1. Field of the Invention
The present invention relates to a vehicle headlight optical axis control unit for carrying out up and down control of the angle of the optical axis of vehicle headlights.
2. Description of Related Art
A vehicle such as an automobile performs longitudinal angular inclinations with respect to the direction parallel to a road because of variations in the number of occupants or a load during stopping, and variations of road conditions during running (the angle of the inclination is referred to as “inclination angle” from now on). At the same time, in accordance with the inclination angle, the optical axis of the headlights fixed to the vehicle varies with respect to the road surface. When the optical axis of the headlights is too upward with respect to the road surface, the light will blind oncoming motorists, whereas when the optical axis of the headlights is too downward, the sight of the driver becomes too narrow, thereby hindering safe driving of the vehicle. In view of this, it becomes essential to carry out the up and down control of the angle of the optical axis of the headlights (called “optical axis angle” from now on) with respect to the running direction of the vehicle in response to the inclination angle.
Up to now, a variety of headlight optical axis control units have been proposed which detect the inclination angle and adjust the optical axis angle. These control units are roughly divided into a dynamic control system and a static control system. The dynamic control system employs the optical axis control that detects the inclination angle during running and adjusts the optical axis angle continuously. In contrast, the static control system carries out the optical axis control only once (usually before the start of the vehicle), without detecting the inclination angle or adjusting the optical axis angle thereafter.
Among the dynamic control system, an optical axis control unit is proposed with the aim of improving the accuracy of the optical axis control and the endurance of the components of the control unit such as an actuator (see, Relevant Reference 1, for example).
The technique disclosed in Relevant Reference 1 places sensors for detecting the amount of displacement from the road surface at the front and rear sides of the vehicle, averages the amounts of displacement detected by the sensors, and calculates the inclination angle from the two average values. Then, it determines the optical axis angle to be adjusted from the thus calculated inclination angle, and carries out the optical axis control. The calculation by the averaging and the optical axis control are carried out as a pair that is performed successively during running of the vehicle.
Among the static control system, an optical axis control unit is proposed which calculates the average value of a plurality of data on the inclination angle during stopping of the vehicle, adjusts the optical axis angle in accordance with the average value during stopping, and fixes the optical axis angle to the adjusted one during running. In this case, the inclination angle is detected by stroke sensors attached to front and rear wheels of the vehicle (see, Relevant Reference 2, for example).
As another static control system, an optical axis control unit is proposed which solves the problem of the inclination angle of a vehicle, which occurs in the time lag from pressing down the accelerator during stopping of the vehicle to the beginning of the running state. It carries out the optical axis control on the basis of the data on the inclination angle at a specified time before the speed sensor detects the start. Thus, it circumvents the actuator drive according to the data on the turned-up inclination angle of the vehicle during the time lag (see, Relevant Reference 3, for example).
Relevant Reference 1: Japanese patent application laid-open No. 10-181424/1998 (pp. 3–5, and FIGS. 1 and 4).
Relevant Reference 2: Japanese patent application laid-open No. 11-105620 (p. 3, and FIGS. 1 and 2) Relevant Reference 3: Japanese patent application laid-open No. 2000-233681 (pp. 4–5, and FIGS. 1 and 3)
The conventional automatic optical axis angle adjusting apparatus for the automobile headlights with the foregoing configuration disclosed in the Relevant Reference 1, which is one of the dynamic control systems, has a limit in the improvement in the endurance or the reduction in the power consumption. It is difficult to reduce the number of operations of the actuator, an optical axis driving means, so that the driving mechanism components constituting the actuator such as a motor and gears are apt to be subjected to wear, thereby presenting a problem of increasing the cost in its entirety.
As for the conventional optical axis adjusting apparatus for the vehicle headlights disclosed in the Relevant Reference 2 and the auto-leveling apparatus for the automobile headlamps disclosed in the Relevant Reference 3, they are both the static control system. Accordingly, although they can curb the cost increase caused by the foregoing reasons, they cannot cope with large variations in the inclination angle during running, presenting a problem of the safety during running of the vehicle.
In addition, the Relevant References 1, 2 and 3 employ a method of measuring the inclination angle by placing the level sensors at the front and rear sides of the vehicle and by measuring the level difference between the two locations, or a method of measuring the inclination angle by placing a level sensor at one of the front and rear sides and by calculating the level difference from a reference level. Thus, they are apt to be subjected to a measurement error in the inclination angle due to a warp in the vehicle or to a depression in a tire, which impairs appropriate adjustment of the optical axis angle. In addition, depending on the types of the vehicle, they must include a dedicated sensor mounting component and a dedicated control unit, presenting a problem of increasing the cost.